Segregated Witness, usually shortened to SegWit, activated on Bitcoin mainnet at height 481,824. Bitcoin Core records that activation height in its network parameters. The corresponding block header is dated August 24, 2017, in UTC.
What was separated?
BIP 141 defines a witness structure for data used to prove that spending is authorized. A transaction’s conventional identifier excludes this witness, while a separate witness transaction identifier commits to it. The witness is still committed to the block and checked by upgraded validating nodes. Read BIP 141.
This distinction addresses forms of transaction malleability: changes to signing data that could otherwise alter a transaction’s identifier. Stable identifiers are useful when constructing transactions that depend on earlier, not-yet-confirmed transactions.
Capacity measured as weight
SegWit also introduced block weight, with a limit of four million weight units. Base data and witness data count differently. Describing this as every block simply becoming “four megabytes” would conceal the accounting rule and the effect of different transaction mixes. The specification.
A technical milestone with a wider history
The rules describe what the upgrade does. They do not, by themselves, explain every participant’s motives in the wider scaling debate. This entry focuses on the documented activation and mechanism; it is not a complete account of that political history.
Continue with Lightning payment channels for one example of why transaction construction beyond a single payment matters.